Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A yaw-rate sensor for determining a Coriolis force includes a
semiconductor substrate, a mass body mounted so it is movable over the
semiconductor substrate, a drive unit for setting the mass body into an
oscillating movement, and a detection unit for determining a deflection
of the mass body which is caused by the Coriolis force. The detection
unit includes a piezoresistive element, whose electrical resistance is a
function of the deformation of the piezoresistive element.

Claims:

1. A yaw-rate sensor for determining a Coriolis force, comprising: a
semiconductor substrate; a mass body mounted so it is movable over the
semiconductor substrate; a drive unit for setting the mass body into an
oscillating movement; and a detection unit for determining a deflection
of the mass body caused by the Coriolis force, the detection unit
including a piezoresistive element.

2. The yaw-rate sensor according to claim 1, wherein the mass body
includes two mass elements, which are set into movements in opposite
directions by the drive unit, and a piezoresistive element is situated
between the mass elements and is operationally linked to the mass
elements in such a way that deflections of the mass elements caused by
the Coriolis force act in opposite directions on the piezoresistive
element.

3. The yaw-rate sensor according to claim 1, wherein a piezoresistive
element is operationally linked to the semiconductor substrate and a
lever element, which is mounted in an anchor point so it is rotatable on
a semiconductor substrate, is provided, and which is coupled to the mass
body and the piezoresistive element so that a force or movement
introduced by the mass body into the lever element is transmitted by the
lever element using a conversion factor to the piezoresistive element.

4. The yaw-rate sensor according to claim 2, wherein a lever element is
associated with each mass element, and the piezoresistive element is
situated between the lever elements.

5. The yaw-rate sensor according to claim 4, wherein at least one lever
element has an isolating element for electrical isolation, so that a part
of the lever element and the piezoelectric element are electrically
isolated from the semiconductor substrate.

6. The yaw-rate sensor according to claim 2, wherein a lever element is
associated with each mass element, the lever elements are coupled to one
another using a compensating spring, and the piezoresistive element is
situated between one of the lever elements and a suspension point on the
semiconductor substrate.

7. The yaw-rate sensor according to claim 6, wherein a second
piezoresistive element, which is situated between a second lever element
and a second suspension point, is provided and an electrical subtraction
unit is provided, in order to compare signals provided by the
piezoresistive elements.

9. The yaw-rate sensor according to claim 1, wherein the drive unit and
the detection unit are configured in such a way that a drive voltage of
the drive unit is separated with respect to time and/or by differing
frequency modulation from a measuring current of the detection unit.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to a yaw-rate sensor. In particular,
the present invention relates to a micromechanical yaw-rate sensor.

BACKGROUND INFORMATION

[0002] Micromechanical yaw-rate sensors may be used in order to determine
forces and accelerations, for example, in a yaw-rate sensor. A mass body,
which is movable in relation to a substrate along two axes perpendicular
to one another, is provided in one variant. The mass body is set into an
oscillating movement in one direction using a drive unit. If the yaw-rate
sensor is rotated around an axis which is perpendicular to the plane in
which the mass body is movable, the mass body is deflected in this plane
in a direction which is perpendicular to the driven direction. This
deflection is caused by the Coriolis force and may be recorded using a
suitable detection unit, in order to provide a signal which is a function
of a yaw rate of the yaw-rate sensor.

[0003] Various configurations are known for both the drive unit and the
detection unit. The present invention is based on the object of
specifying a yaw-rate sensor which has a reduced space requirement.

SUMMARY OF THE INVENTION

[0004] According to the present invention, a yaw-rate sensor for
determining a Coriolis force includes a semiconductor substrate, a mass
body mounted so it is movable over the semiconductor substrate, a drive
unit for setting the mass body into an oscillating movement, and a
detection unit for determining a deflection of the mass body caused by
the Coriolis force. The detection unit includes a piezoresistive element,
whose electrical resistance is a function of the deformation of the
piezoresistive element.

[0005] In comparison, for example, to a capacitive determination unit for
the position of the mass body, installation space may thus be saved, so
that the yaw-rate sensor may be reduced in size overall. Production
outlay may thus be reduced and the resulting yaw-rate sensor may be
usable more universally due to its reduced external dimensions.

[0006] Preferably, the mass body includes two mass elements, which are set
into movement in opposite directions by the drive unit, and the
piezoresistive element is situated between the mass elements and is
operationally linked to the mass elements in such a way that the
deflections of the mass elements caused by the Coriolis force act on the
piezoresistive element in opposite directions. This allows a differential
measurement via which interfering influences, which influence the
movements of both mass elements in the same way, may be able to be
compensated against one another.

[0007] The piezoresistive element may be operationally linked to the
semiconductor substrate and the yaw-rate sensor may include a lever
element which is mounted in an anchor point so it is rotatable on a
semiconductor substrate and which is coupled to the mass body and the
piezoresistive element so that a force or movement introduced by the mass
body into the lever element is transmitted by the lever element with a
conversion factor to the piezoresistive element. By selecting a
corresponding conversion factor, an adaptation may be performed between
the structural geometry of the moving parts of the yaw-rate sensor and a
signal provided by the piezoelectric element, which is a function of a
force or a deformation of the piezoelectric element.

[0008] A lever element may be associated with each mass element, the
piezoresistive element being situated between the lever elements. A
differential measurement may thus be simplified.

[0009] An isolation element for electrical isolation may be situated on at
least one of the lever elements, so that a part of the lever element and
the piezoelectric element are electrically isolated from the
semiconductor substrate. An interaction between the current path, via
which the signal provided by the piezoelectric element is tapped, and the
drive unit or other moving elements of the yaw-rate sensor may thus be
prevented, whereby the measurement precision may be increased.

[0010] A lever element may be associated with each mass element, the lever
elements being coupled to one another using a compensating spring and the
piezoresistive element being situated between one of the lever elements
and a suspension point on the semiconductor substrate. This represents a
further possibility for decoupling the measuring signal from elements of
the yaw-rate sensor which may interact with the measuring signal.

[0011] A second piezoresistive element, which is situated between a second
lever element and a second anchor point, may be provided, an electrical
subtraction unit being provided in order to compare the signals provided
by the piezoresistive elements. This specific embodiment suggests itself
to simulate a mechanical subtraction, which is used to absorb errors
which affect two mass elements inversely.

[0012] The drive unit may set the mass body into movement mechanically,
electrostatically, magnetically, optically, piezoelectrically,
chemically, and/or thermally. Usability of the piezoresistive element is
independent of the drive principle of the mass body, so that the yaw-rate
sensor is implementable using any arbitrary known drive unit.

[0013] The drive unit and the detection unit may be configured in such a
way that a drive voltage of the drive unit is isolated in time and/or by
a differing frequency modulation from a measuring current of the
detection unit. Mutual influences between the drive voltage and the
measuring current may thus be minimized and the determination may be
improved. A square-wave, triangular, or sinusoidal signal may be used for
the modulation, for example.

[0019]FIG. 1 shows a micromechanical yaw-rate sensor 100. Yaw-rate sensor
100 is situated on a semiconductor substrate 105. An x-y-z coordinate
system shown on the lower left is used for reference. The object of
yaw-rate sensor 100 is to provide an electrical signal, which is a
function of a rotation of yaw-rate sensor 100 around the z axis.

[0020] Yaw-rate sensor 100 includes a left section and a right section,
which are constructed in a mirror image to one another with respect to a
perpendicular central axis, the left section being described hereafter. A
rectangular mass body 115 is fastened using coupling springs 120 on a
frame 125. Frame 125 is also rectangular and is closed on three sides. A
fourth side points in the direction of the central axis and is
interrupted in its center. Coupling springs 120 lead from the corners of
mass body 115 in the x direction to fastening points on frame 125. Each
of coupling springs 120 is curved essentially in a U-shape in the y
direction, the bends pointing toward one another in the y direction, so
that coupling springs 120 are elastically deformable in the event of a
movement of mass body 115 with respect to frame 125 in the x direction
and are rigid in the event of a movement of mass body 115 in the y
direction. Therefore, mass body 115 is only movable in the x direction,
but not in the y direction, in relation to frame 125.

[0021] Oscillation springs 130 are attached at the corners of frame 125,
the oscillation springs leading in the y direction to anchor points 110,
which are mechanically connected to semiconductor substrate 105, but are
electrically isolated from semiconductor substrate 105. Oscillation
springs 130 are constructed corresponding to coupling springs 120, the
U-shaped bends of oscillation springs 130 running in the x direction,
however, so that frame 125 is movable in the y direction and is immovable
in the x direction in relation to substrate 105. As a whole, mass body
115 is thus suspended so it is movable in the x-y plane.

[0022] Analysis springs 135 are fastened on the side of mass body 115
which faces toward the axis of symmetry. Analysis springs 135 extend in
the x direction through the interruption of the fourth side of frame 125
up to a pair of levers 140 running in the y direction. Levers 140 are bar
shaped and lie on an axis in the y direction. Analysis springs 135 are
bent in S-shapes or Z-shapes and are therefore rigid in the x direction
and elastic in the y direction. Fastening points of analysis springs 135
on levers 140 are located at the ends of levers 140 which face toward one
another in the y direction, i.e., the lower end of upper lever 140 and
the upper end of lower lever 140.

[0023] Upper lever 140 is mounted in the area of its upper end using a
bending web 147, which runs in the x direction, on an anchor point 145,
which is attached in an electrically isolated manner on substrate 105.
Bending web 147 is dimensioned narrower than lever 140, so that lever 140
is deflectable around a pivot point in the area of bending web 147. Lower
lever 140 is constructed correspondingly to upper lever 140, lower lever
140 being mounted in the area of its lower end using a bending web 147 on
an anchor point 145. Upper and lower bending webs 147 run parallel to one
another, in each case to the left from levers 140. Bending webs 147 may
also be omitted depending on the selected specific embodiment, and levers
140 may be connected directly to anchor points 145.

[0024] A piezoresistive element 150, which extends to the right in the y
direction, is fastened on lower lever 140 in an area between bending web
147 and the lower end. A piezoresistive element 150 corresponding
thereto, which also extends to the right in the y direction and runs
parallel to upper piezoresistive element 150, is located on lower lever
140. Forces and movements which are introduced into lever 140 by analysis
spring 135 are converted into forces or movements which act on
piezoresistive element 150 in a known way according to the lever rule and
are based on a conversion factor, which corresponds to a ratio of
distances of the particular fastening points of analysis spring 135 and
piezoresistive element 150 from the pivot point of lever 140.

[0025] The right section of yaw-rate sensor 100, which forms a mirror
image in relation to the central axis, is located to the right of
piezoresistive elements 150.

[0026] The right ends of piezoresistive elements 150 are fastened to
levers 140 of the right section. Piezoresistive elements 150 deliver a
voltage signal which is a function of the force acting in the x direction
thereon. Through a corresponding selection of the positions of
piezoresistive elements 150 in the y direction on levers 140, the
relationship of the voltage signal to the deflection or the deflection
force of analysis springs 135, and therefore of frame 125, in the x
direction may be set.

[0027] Piezoresistive elements 150 may be selected from manifold known
constructions. For example, a solid material may be used, such as a bar
made of silicon, silicon carbide, or a polymer. Nanowires or carbon
nanotubes may also be used. Furthermore, a resistance change of material
on a surface of piezoresistive element 150 may be induced, for example,
by a metal thin film applied to a bar or by doping which diffuses into
the bar. In still a further specific embodiment, piezoresistive element
150 may be applied to an isolating bar, for example, made of silicon
dioxide, as a strain gauge.

[0028] Levers 140 are one-sided levers which have a step-down ratio, which
is a function of a ratio of the lengths between the introduction and
delivery points of forces and anchor point 145. This ratio is arbitrarily
settable by displacing the piezoresistive elements toward or away from
one another. If piezoresistive element 150 is located at half of the
length between an anchor point 145 and an analysis spring 135, the ratio
is 1:2. Two-sided levers 140 may also be used in further specific
embodiments.

[0029] Symbolically shown drive units 155 are configured for the purpose
of moving frame 125 in the y direction. Drive units 155 may be
implemented in manifold known ways. In one specific embodiment, drive
unit 155 is implemented as electrostatic, for example, using plate
structures or fingers or comb structures ("interdigital fingers"). By
applying a voltage between opposing or interlocking fingers or plates, an
attractive force acts between these elements, which causes a deflection
of frame 125. Alternatively, drive unit 155 may also act magnetically and
use the movement force based on the Lorentz force to deflect frame 125,
in that areas which have current flowing through them perpendicularly to
a magnetic field exert a force on frame 125. In further specific
embodiments, drive unit 155 may cause the force piezoelectrically or
thermally.

[0030] Drive units 155 set frames 125 into a sinusoidal oscillation in the
y direction, the phases of the movements of both frames 125 being
phase-shifted with respect to one another by 180°. Mass bodies 115
are also set into a sinusoidal oscillation in the y direction by the
fastening using coupling springs 120. This deflection is symbolized by
dashed arrows in the y direction. Because of Coriolis forces, in the case
of a rotation of yaw-rate sensor 100 around the z axis, each mass body
115 is deflected in the x direction. This deflection is symbolized by
solid lines in the x direction. The deflections of mass bodies 115 in the
x direction are phase-coupled to the movements of mass bodies 115 in the
y direction, which are caused by drive units 155. Deflection directions
of both mass bodies 115 are antiparallel, i.e., toward or away from one
another, depending on the direction of the rotation around the z axis.

[0031] The detection oscillation of mass body 115 in the x direction may
be used for position feedback control ("closed loop operation") via a
symbolically shown control unit 160, e.g., using electrostatic forces.
The detection movement in the x direction is thus advantageously reduced
and the characteristic curve of yaw-rate sensor 100 is thus linearized,
i.e., the mechanical nonlinearities of yaw-rate sensor 100 are
suppressed. The position control may alternatively be performed with
respect to semiconductor substrate 105 or frame 125.

[0032] Mass body 115 represents an oscillating system mounted at multiple
points, the oscillation capability in the x direction (drive mode) and in
the y direction (detection mode) typically not corresponding to one
another because of variations during manufacturing of yaw-rate sensor
100. To compensate for the natural frequencies, one of them is typically
influenced so that the frequencies coincide. This is preferably brought
about by a force in an electrostatic field, for which purpose
corresponding electrodes may be designed on frame 125 and on mass body
115. In one specific embodiment, the modes may be influenced by control
unit 160. In another specific embodiment, yaw-rate sensor 100 is
accordingly calibrated during the manufacturing, for example, by laser
trimming. Through the influencing, the detection oscillation may be made
to have a higher-order resonance in the x direction and therefore the
signal-to-noise ratio may be improved. This is referred to as full
resonance tuning.

[0033]FIG. 2 shows yaw-rate sensor 100 from FIG. 1 having a single
piezoresistive element 150. The illustration corresponds to that from
FIG. 1 with the difference that only two levers 140 are provided, between
which only one piezoelectric element 150 is situated. Piezoelectric
element 150 is connected at each of its ends to one free end of one of
levers 140. Free levers 140 are situated essentially parallel to one
another. Each of levers 140 is coupled via a compensation spring 135 to
one of mass bodies 115. Piezoelectric element 150 is situated parallel to
a central axis.

[0034] Drive unit 155 in FIGS. 1 and 2 may be activated using an
approximately sinusoidal or square-wave alternating drive voltage. In
general, the drive voltage interferes with the measuring procedure using
piezoelectric element or elements 150. In order to achieve signal
isolation of drive and detection, various procedures are possible.

[0035] In a first variant, a time multiplexing method may be used. For
this purpose, a measuring pulse may be fed into piezoresistive element
150 in the half phases in which no drive voltage is applied to drive unit
155, and the forces acting on piezoresistive element 150 may be measured.
In a second variant, the measuring current may be modulated by
piezoresistive element 150 using a different frequency than the drive
frequency, so that the two signals are separable from one another by
frequency filters. In a third variant, drive units 155 may be
electrically isolated from remaining yaw-rate sensor 100. For this
purpose, isolation trenches may be introduced into semiconductor
substrate 105, which may be filled using an isolating material, for
example, an oxide.

[0036] A third variant is shown in FIG. 3. FIG. 3 shows yaw-rate sensor
100 according to FIG. 1, each of levers 140 being electrically
interrupted by an isolating element 310 but simultaneously being
mechanically connected, so that a current path runs through each of
piezoresistive elements 150 between associated anchor points 145,
isolated from remaining yaw-rate sensor 100. Isolating element 310 is
preferably implemented as an isolation trench in lever 140. An electrical
connection to an analysis circuit is performed via anchor points 145,
which are electrically isolated from semiconductor substrate 105.

[0037] In a fourth variant, which is shown in FIG. 4, the current paths
leading through piezoresistive element 150 are selected in such a way
that they do not run through any element of yaw-rate sensor 100 which is
electrically connected to a drive voltage of drive unit 115. FIG. 4 shows
yaw-rate sensor 100 from FIG. 2, piezoresistive elements 150 each being
situated between one of levers 140 and one suspension point 410,
suspension points 410 being fastened in an electrically isolated manner
on semiconductor substrate 105. A compensation spring 420, which is
movable in the x direction and is rigid in the y direction, is situated
between levers 140. In addition, each lever 140 is connected using one of
analysis springs 135 to one of mass elements 115. Bending webs 147 are
situated parallel to levers 140.

[0038] If a mechanical subtraction of the movements of mass bodies 115 is
not performed in the x direction in the view shown in FIG. 4, electrical
signal processing corresponding to FIG. 5 is necessary. FIG. 5 shows an
electrical signal mixer 500 for the yaw-rate sensor from FIG. 4. Signal
mixer 500 includes a so-called Wheatstone bridge, which is constructed
from both piezoresistive elements 150 from FIG. 4 and two measuring
resistors 510, and a subtractor 520. One measuring resistor 510 and one
piezoresistive element 150 are connected in series in each case to a
constant measuring voltage UM. Subtractor 520 electrically
calculates a difference between the voltages which result in each case
between one of measuring resistors 510 and piezoresistive element 150
associated therewith. By varying one of measuring resistors 150, the
signal mixer may be tuned to compensate for imperfections of the
mechanical construction of yaw-rate sensor 100.